Odd-nosed monkey scapular morphology converges on that of arm-swinging apes

Odd-nosed monkeys 'arm-swing' more frequently than other colobines. They are therefore somewhat behaviorally analogous to atelines and apes. Scapular morphology regularly reflects locomotor mode, with both arm-swinging and climbing anthropoids showing similar characteristics, especially a mediolaterally narrow blade and cranially angled spine and glenoid. However, these traits are not expressed uniformly among anthropoids. Therefore, behavioral convergences in the odd-nosed taxa of Nasalis, Pygathrix, and Rhinopithecus with hominoids may not have resulted in similar structural convergences. We therefore used a broad sample of anthropoids to test how closely odd-nosed monkey scapulae resemble those of other arm-swinging primates. We used principal component analyses on size-corrected linear metrics and angles that reflect scapular size and shape in a broad sample of anthropoids. As in previous studies, our first component separated terrestrial and above-branch quadrupeds from clambering and arm-swinging taxa. On this axis, odd-nosed monkeys were closer than other colobines to modern apes and Ateles. All three odd-nosed genera retain glenoid orientations that are more typical of other colobines, but Pygathrix and Rhinopithecus are closer to hominoids than to other Asian colobines in mediolateral blade breadth, spine angle, and glenoid position. This suggests that scapular morphology of Pygathrix may reflect a significant reliance on arm-swinging and that the morphology of Rhinopithecus may reflect more reliance on general climbing. As 'arm-swinging' features are also found in taxa that only rarely arm-swing, we hypothesize that these features are also adaptive for scrambling and bridging in larger bodied anthropoids that use the fine-branch component of their arboreal niches.